In a digital radio communication system, an interference canceller technology, which estimates a desired signal and interference signal based on maximum likelihood estimation, is used. There are interference canceller systems in a CDMA system such as a single user (SUD: Single User Detection) type and multi-user (MUD: Multi User Detection) type.
As the MUD, there are a multi-stage type interference canceller that improves a reception characteristic by repeating a plurality of times (multi-stage) processing on the receiving side of generating interference replica signals of other users based on channel estimated values and decision data, subtracting these replica signals from a received signal and thereby improving an SIR (Signal to Interference Ratio), and a single-stage type interference canceller that improves a reception characteristic by applying ranking processing to likelihood of all symbols of all channels, generating replica signals for symbols in descending order of likelihood and subtracting the replica signals from a received signal on the receiving side and thereby improving an SIR.
A W-CDMA (Wideband-Code Division Multiple Access) of a digital radio communication system is a system suitable for implementing a multimedia communication handling various transmission rates. Technologies concerning an application of an interference canceller to this W-CDMA currently being developed and published by research organizations are technologies mainly applied to an uplink DPCH (Dedicated Physical CHannel).
That is, in an application of an interference canceller to W-CDMA, this technology creates replica signals from a received signal of DPDCH (Dedicated Physical Data CHannel) or DPCCH (Dedicated Physical Control CHannel) of other stations and subtracts those replica signals from a total received signal and thereby cancels interference components from the received signals through demodulation at the own station. This technology is intended to mainly implement a reduction of required Eb/No (SIR) of DPDCH by applying an interference canceller (MUD) to a base station.
Even if replica signals are created using either DPDCHs or DPCCHs of other stations, this technology cancels interference among other stations from the received signal only through demodulation of DPDCHs. This is because delays involved in the processing by an interference canceller of creating replica signals and subtracting the replica signals from the received signal can be tolerated for DPDCHs more or less (about several frames).
For an interference canceller technology such as MUD, reducing not only an amount of processing but also a processing delay is an important issue in implementation. An actually reported demodulation delay of DPDCH (data section) by an interference canceller is on the order of several slots to several frames.
On the other hand, an allowable amount of delay of signals transmitted with DPCCH is restricted a great deal. For example, in the case where a slot configuration of an uplink signal is determined as shown in FIG. 1, that is, DPDCH is assigned to an in-phase component (Ich) and DPCCH is assigned to a quadrature component (Qch), it is stipulated for standardization as shown in FIG. 2 that the receiving side should change transmission power (indicated by arrow X in FIG. 2) from the first pilot signal immediately after a TPC bit is received according to a TPC (Transmission Power Control) bit for transmission power control. This allows a processing delay of only several tens of μs. Furthermore, only a processing delay of not more than 1 slot is allowed for demodulation of FBI (FeedBack Information) which is a command for transmission diversity or SSDT (Site Selection Diversity Transmission) or TFCI (Transport-Format Combination Indicator) indicating the type of communication quality such as a transmission rate and service depending on the purpose of use.
Therefore, demodulation of a DPCCH signal with such a small amount of allowable processing delay needs to be processed before canceling interference or in the middle of cancellation of interference. Thus, unlike DPDCH, it is difficult to implement an improvement of a DPCCH reception characteristic by an interference canceller, that is, a reduction of a required SIR or Eb/No, etc.
When such an interference canceller is introduced to a base station, interference with the DPDCH is reduced as shown in FIG. 3, and therefore it is possible to reduce transmission power of the DPDCH at the communication terminal and reduce interference with other stations. Thus, by reducing the transmission power of the DPDCH it is also possible to reduce the transmission power of the DPCCH.
However, utilizing the effect (area indicated by broken line in FIG. 3) resulting from a reduction of the transmission power of the DPDCH at the communication terminal for increasing the system capacity, that is, utilizing the effect for additions of new users will result in an increase of interference corresponding to the additional users, which requires relatively more transmission power of the DPCCH, preventing transmission power required for DPCCHs with a restricted demodulation delay such as channel estimation and demodulation of a TPC command from reducing.
Thus, as opposed to again factor (G) in transmitting DPDCH multiplexed with DPCCH when no interference canceller is introduced, this gain factor changes a great deal when an interference canceller is introduced to a base station.
An optimal gain factor depends on the capability of an interference canceller, that is, interference cancellation performance, applicable channels (e.g., only applicable to a communication channel (DPCH) at a specific transmission rate), applicable parts (e.g., applied to only DPDCH). Therefore, the optimal gain factor differs a great deal between a base station to which no interference canceller is applied and a base station to which an interference canceller is applied. In addition, the optimal gain factor may also differ between base stations using interference cancellers of different capabilities.
By the way, it is stipulated by 3GPP (3rd Generation Partnership Project) that the value of this gain factor should be determined on the network side (upper layer) and transmitted to the communication terminal side by a control signal.
On the other hand, transmission power control (power control) is constructed of an inner loop which is controlled by a base station using a target SIR as a base and an outer loop in which an RNC (Radio Network Controller) controls a target SIR using channel quality (bit error rate (BER) or a block error rate (BLER) as a base.
The value of the target SIR controlled by the RNC through the outer loop during diversity handover (Diversity Hand Over: DHO) is controlled as a value common to a plurality of base stations and as the only value. This is because there is a premise that the SIR value satisfying required quality (BLER, etc.) does not vary drastically among a plurality of base stations.
During diversity handover, a communication terminal communicates with two base stations simultaneously. As described above, an optimal gain factor differs when the communication terminal is communicating with a base station with no interference canceller and when communicating with a base station with an interference canceller. Therefore, how to determine an optimal gain factor during diversity handover when there is a difference between base stations with and without an interference canceller or between base stations with interference cancellers of different capabilities is a question, but it is an actual situation that there is no technique to solve this problem.
Furthermore, when an interference canceller is introduced to a base station, the reception capability with respect to a target SIR, that is, a relationship between the SIR of DPCCH and the quality of DPDCH (BER, BLER) is changed. Therefore, even if the measured value of the SIR of DPCCH is constant, the channel quality of DPDCH may vary depending on the demodulation capability (basic reception capability, presence/absence of an interference canceller or its interference cancellation capability, etc.) of DPDCH of each base station.
Therefore, keeping the same relationship between the SIR of DPCCH and the quality of DPDCH (BER, BLER) for when an interference canceller is introduced to a base station and when no interference canceller is introduced to a base station requires the gain factor to be adjusted according to whether or not to use an interference canceller or the capability of each interference canceller.
However, during diversity handover, a communication terminal sends a signal with DPDCH and DPCCH multiplexed made up of a common gain factor. Thus, it is unavoidable that the relationship between the SIR of DPCCH and the quality of DPDCH (BER, BLER) will vary from one base station to another that receives the signal.